Currently, large quantities of edible fats and oils as mentioned above are used in our country. Although a certain part of spent oils (aforementioned waste edible oils) is recycled as a raw material for producing soap, for example, they are mostly conveyed to a refuse dump without being recovered to be burned together with other combustible wastes or disposed in landfills together with incombustible wastes in actuality.
It has been known that a fatty acid alkylester could be obtained by subjecting major ingredients of fats and oils such as monoglyceride, diglyceride and triglyceride, to a transesterification with an alkyl alcohol (Handbook of Organic Chemistry, published by Gihodo Shuppan, 1988, pp. 1407-1409). Various attempts using the transesterification have thus far been made to produce an alkylester usable as a diesel engine fuel oil from the aforementioned oils and fats, as disclosed in Japanese Unexamined Patent Publication Nos. 7-197047 and 7-310090, for example. It has not been possible, however, to obtain any alkylester complying with current requirements of the Japanese Industrial Standards (JIS) imposed on light oil products by such prior art technology.
In a typical industrial process for a production of a fatty acid ester from triglyceride, in particular triglyceride contained in natural fats and oils, a fatty acid glyceride is caused to react with a lower alcohol in the presence of a metal alkali catalyst at normal pressure and at a temperature near the boiling point of the lower alcohol. The reaction process, however, uses the metal alkali catalyst dissolved in the reaction solution. Therefore, the metal alkali catalyst remains dissolved in the liquid reaction product obtained by this reaction process, resulting in a problem that it is difficult to separate and recover the metal alkali catalyst from the liquid reaction product.
There are another problems as follows. Natural fats and oils generally contain a large amount of free, i.e., unesterified, fatty acids. The amount of free fatty acids depends on origins of the raw material fats and oils and processes of them. On the average, it is more than 3 weight % of the fats and oils. When a metal alkali catalyst is used in the reaction solution having a large amount of free fatty acids, a fatty acid soap may be generated as a by-product of the transesterification, resulting in the need of an excessively large amount of the metal alkali catalyst. Alternatively,the fatty acid soap makes it difficult to separate a fatty acid ester layer from a glycerin layer in the liquid reaction product. To solve these problems, a pre-treatment process for removing the free fatty acids from the reaction solution is necessary for a transesterification of a fatty acid glyceride with an alcohol in the presence of the metal alkali catalyst.
To solve the above-mentioned problems, Japanese Unexamined Patent Publication No. 61-14044 discloses a process of transforming the free fatty acids into a fatty acid ester in the presence of an acid catalyst as the pre-treatment process for removing the free fatty acids. That is, in this process, the free fatty acids are pre-treated to be transformed into an ester, followed by a transesterification of a fatty acid glyceride in the presence of a metal alkali catalyst. However, when the transesterification is performed with the acid catalyst, which has been used in the pre-treatment process, remaining in the reaction solution, a part of the metal alkali catalyst to be used for the transesterification is neutralized by such a remained acid catalyst. Thus, there exists a problem that such a transesterification requires an increased amount of the metal alkali catalyst to compensate for the neutralized part of the metal alkali catalyst.
In addition, Japanese Unexamined Patent Publication No. 6-313188, for example, suggests a process for producing a fatty acid ester in which a solid acid catalyst is used to eliminate the need of the above-mentioned pre-treatment process. However, acid catalysts including the solid acid catalyst have a serious drawback, that is, their catalytic activity is lower than that of metal alkali catalysts in the transesterification of fats and oils. Thus, there exists a problem that the transesterification using acid catalysts needs a larger catalyst amount.
The present invention is aimed at overcoming the aforementioned problems residing in the prior art technology. It is therefore an object of the invention to provide a process for preparing or producing a fatty acid alkylester from fats and oils, in which it is possible to eliminate the pre-treatment process for esterifying the free fatty acids contained in the fats and oils and, by suppressing the by-production of a fatty acid soap, to eliminate or simplify recovering and refining processes of the reaction product.